FIG. 10 is a perspective view illustrating a prior art pulsation laser. FIG. 11 is a sectional view of a part of the pulsation laser shown in FIG. 10. In these figures, reference numeral 101 designates a first conductivity type semiconductor substrate. A first conductivity type lower cladding layer 102 is disposed on the substrate 101. An active layer 103 is disposed on the lower cladding layer 102. An upper cladding layer 104 of a second conductivity type, opposite the first conductivity type, is disposed on the active layer 103. A first electrode 106b is disposed on the rear surface of the substrate 101 and a second electrode 106a is disposed on the upper cladding layer 104. Reference numeral 105 designates a first conductivity type current blocking layer.
A description is given of the operation. As shown in FIG. 11, current injected into the laser structure from the electrode 106a is concentrated by the current blocking layer 105, somewhat broadened in the upper cladding layer 104 between the current blocking layer 105 and the active layer 103, and injected into the active layer 103. Thereby, light is generated in a region of the active layer 103 where current flows, i.e., a region other than the region directly under the current blocking layer 105, resulting in laser oscillation.
In the active layer 103, the extent of the region where light exists and the extent of the region where current flows in the active layer 103 vary according to the thickness d of the upper cladding layer 104 between the current blocking layer 105 and the active layer 103. When the thickness d is appropriately selected, the extent of the region where light exists becomes larger than the extent of the region where current flows, i.e., a region 110 where no current flows but laser light exists is produced in the active layer 103. This region 110 is called a super-saturated absorption band. In the super-saturated absorption band, light is absorbed until the quantity of electrons and holes generating light reaches a certain value. When the quantity of electrons and holes exceeds that value, electrons and holes which have been stored in that region are output light for a time, and an initial value is attained. That is, the super-saturated absorption band causes intermittent laser oscillation, whereby so-called pulsation laser emission is realized.
In the prior art semiconductor pulsation laser utilizing the super-saturated absorption band, however, both of the region where light exists and the region where current exists are controlled only by the thickness d of the cladding layer between the active layer and the current blocking layer to produce the super-saturated absorption band. Therefore, the respective extent of each region is set at compromised values than at optimum values, and the permissible extent for each of these regions is usually very narrow, resulting in difficulties in design and fabricating of the laser.
Furthermore, in the prior art semiconductor pulsation laser, the frequency of the intermittent laser oscillation varies according to various factors, such as the doping of the respective layers. Therefore, it is very difficult to design a semiconductor pulsation laser, and the trial manufacture must be repeated to attain a desired frequency.